The use of polymers, particularly poly(alpha-olefins) or copolymers thereof, to reduce the drag of a hydrocarbon flowing through a conduit, and hence the energy requirements for such fluid hydrocarbon transportation, is well known. These drag reducing agents or DRAs have taken various forms in the past, including slurries or dispersions of ground polymers to form free-flowing and pumpable mixtures in liquid media. A problem generally experienced with simply grinding the polyalpha-olefins (PAOs) is that the particles will “cold flow” or stick together after the passage of time, thus making it impossible to place the PAO in the hydrocarbon where drag is to be reduced, in a form of suitable surface area, and thus particle size, that will dissolve or otherwise mix with the hydrocarbon in an efficient manner. Further, the grinding process or mechanical work employed in size reduction tends to degrade the polymer, thereby reducing the drag reduction efficiency of the polymer.
In general, the DRA polymer may be obtained via solution polymerization of an alpha olefin monomer, or a mixture of olefinic monomers, or from bulk polymerization (i.e., without solvent) of such monomer(s). The DRA polymer may then be subsequently made into particulate form by cutting, chopping, granulating, grinding and/or other size reduction, at cryogenic or ambient temperatures. Alternatively, it may be precipitated from solution by addition of a non-solvent component. Mixtures of polymer solids from both sources may be used.
Once the polymer DRA is prepared and reduced to appropriate particulate form, it may be incorporated with a liquid carrier to form a slurry. In some embodiments the liquid carrier is a non-solvent for the polymer DRA and its selection may vary widely. A problem that is often encountered, however, is that there is a natural tendency for such slurries to settle over time, or to separate or agglomerate such that the slurries no longer maintain a free-flowing and pumpable nature.
Some ways of addressing this problem have been to include (1) a partitioning agent, (2) a wetting agent, and/or (3) a rheology modifier in the slurry together with the carrier liquid and the DRA polymer particles. These three components, which are frequently all included, may be referred to generally as “suspension aids”. The purpose of the partitioning agent is to physically hold the polymer DRA particle surfaces apart. The purpose of the wetting agent is to wet the polymer DRA surface, and the purpose of the rheology modifier is to increase the viscosity of the liquid carrier to slow down polymer DRA particle settling or rising. In some cases a single ingredient may serve more than one purpose within the suspension aid package. In one non-limiting embodiment herein a “suspendsion” is defined as a stable slurry.
The liquid carrier is, in some embodiments, a non-solvent for the polymer DRA and may vary widely. Selections for this component may include both aqueous and non-aqueous liquids, including, for example, water and aqueous solutions of various pH and ionic strength; alcohols and fatty alcohols; glycols and diols; glycol ethers; glycol esters; mixtures of these; and the like.
The wetting agent is often included in such formulations, particularly for aqueous carrier liquids. Without a wetting agent the liquid carrier would quickly drain away from a non-wetted polymer surface. This would result in a highly separated suspension. Fatty acid waxes have been used as wetting agents, as well as commercially available surfactants, including, but not necessarily TWEEN™, SPAN™, BRIJ™, and MYRIJ™. These surfactants, which are generally sorbitan esters, ethoxylated sorbitan esters, alcohol ethoxylates and polyoxyethylene fatty acids, are available from Uniqema. Other surfactants that have been or may be employed include, but are not limited to, TERGITOL™ and TRITON™ surfactants available from Dow Chemical Company, IGEPAL™ ethoxylated alkylphenol surfactants available from Rhodia HPCII, and the like.
Inclusion of a partitioning agent may also be generally desirable in these slurries. This is because polymer DRAs are often in the form of soft, tacky particles that will agglomerate, or cold flow, when their unaltered surfaces come into contact with one another. Common partitioning agents employed may include, but are not necessarily limited to, fatty acid waxes, stearic acid and stearate salts (e.g. alkali earth metal stearates such as calcium stearate and magnesium stearate), stearamides, polyolefin homopolymers and copolymers of various densities; oxidized polyethylene; polystyrene and copolymers; carbon black and graphites; micronized polyphenylene sulfide (PPS), polyphenylene oxide (PPO), polyamides, polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyvinyl chloride (PVC); precipitated and fumed silicas; natural and synthetic clays; organo-clays; aluminum oxides; talc; boric acid; polyanhydride polymers; sterically hindered alkyl phenol oxidants; magnesium, calcium and barium phosphates, sulfates, carbonates and oxides; mixtures thereof; and the like.
It should be noted, however, that many partitioning agents require heating to reach maximum effectiveness in a formulation. But heating a process stream is often economically disadvantageous in commercial production. Often the proportion of partitioning agent levels in a final DRA formulation may be as high as 10% to react effectiveness. Since the partitioning agent is a non-active component providing no drag reduction and used only for suspension stability, the more of it that is required in the formulation, the higher the cost the product will be without a commensurate increase in performance.
A rheology modifying agent may, in some embodiments, also be added to minimize settling of the polymer DRA slurry. By adding the modifying agent to the liquid carrier, settling or rising of the DRA polymer may be hindered or prevented. Common rheology modifying agents include, but are not necessarily limited to, polysaccharides and natural gums, cellulosics, natural or modified starches, synthetic polymers such as polyvinyl alcohol (PVA), polyethylene oxide (PEO), polyethylene glycol (PEG), polyvinyl pyrrolidone (PVP), and the like. However, these materials generally have no particulate properties that would permit them to serve as a partitioning aid. Rheology modifiers for aqueous suspensions typically require sufficient hydration in solution so that hydrogen bonding between the polymer chains of the rheology modifiers provides for weak, temporary inter-chain bonding. This interaction gives rise to a viscosity increase within the aqueous solution. The use of glycols and other freeze protectants as components in aqueous carriers typically interfere with the rheology modifier and disable its function and usefulness.
Another practice to enhance suspension stability is to match the suspended particle density to the carrier density by using a combination of carrier components in a proportion such that the overall carrier mixture density equals the suspended particle density. A drawback of this practice is that there are a limited number of carriers available at an economical cost that have both the proper densities and the necessary hydrophobic/hydrophilic properties.
It may be appreciated that considerable resources have been spent on both chemical and physical techniques for easily and effectively delivering drag reducing agents to the fluid that will have its drag or friction reduced. Yet none of these prior methods has proven entirely satisfactory. Thus, it would be desirable if a drag reducing agent could be developed which is stable during storage, transportation and handling, that is, does not separate out the DRA particles. In view of the above, there is still a need in the art to discover ways to produce stabilized polymer DRA slurries that are convenient and economical and which do not unacceptably suffer from the drawbacks discussed hereinabove. In non-limiting examples, it would be helpful to develop a method to produce stable DRA products that avoid the application of additional heat and/or avoid grinding of the suspension aid per se.